I'm about to embarke on my first kiteboard build. I have built variaous landboards and skateboards so i am experienced in the techniques of board building.

My build will be a 142 x 43 mutant.

I'm planning on running 40mm rocker on the back half and 70mm on the front.

Anyway I have one question and its this.

I am going to put 5mm of concave into the board. on my current TT i notice the concave finished at the fins. What is the disadvantage of runing the concave all the way through to the tips?....
i can accomidate a wasting concave if essential. but its more of a pain in the arse building the press this way.

142 too short?. yes i really want it longer. but thats the largest i can fit sideways in the boot of my car. otherwise i have to fold a seat down which becomes and issue with the 3 kids i drive around. .... So i need to stick to 142 or less. Also side ways in my boot means i can have a "secret" stash of kites/boards just incase an unforeseen window of opportunity arises!.

You have your priorities wrong. Put your kids in the boot, so that you have space for your board on the passenger seat, where it belongs.

On topic again. I have 3 Mako's, which I ride exclusively in waves. The Mako's have concave all the way to the tips. The bigger Mako's perform fine when set up in mutant mode.

About 10 years ago, the people I windsurfed with had AHD boards, with concave that started at the front of the board, but washed out to a flat back. So maybe you can compromise on your Mutant with a similar design.

also it can allow for a greater rocker when board is on edge, yet a flatter rocker when board is flat, again, better effciency

build a better rocker table, a flat one, where you can adjust concave.
ie a flat table with a piece of 1/4" MDF on top. Put strips of chipboard under the MDF and voila, you can play with concave all you want. The MDF adjusts for imperfections in your concave chipboard strips